Onset layer for thin film disk with CoPtCrB alloy

ABSTRACT

The thin film disk of the invention includes a thin film pre-seed layer of amorphous or nanocrystalline structure. The pre-seed layer, which may be chromium-tantalum (CrTa) or aluminum-titanium (AlTi) or aluminum-tantalum (AlTa), is deposited prior to a first crystalline layer. Although the pre-seed layer may be amorphous or nanocrystalline, for brevity it will be referred to herein as amorphous which is intended to encompass a nanocrystalline structure. In the preferred embodiment of the present invention, pre-seed layer is sputtered onto a nonmetallic substrate such as glass, followed by a ruthenium-aluminum (RuAl) layer with B2 structure. The use of the pre-seed layer improves grain size and its distribution, in-plane crystallographic orientation and coercivity (Hc) and SNR. In a preferred embodiment of the present invention, the pre-seed layer is followed by the RuAl seed layer, a Cr alloy underlayer, an onset layer and a magnetic layer. The amorphous pre-seed layer also allows use of a thinner RuAl seed layer which results in smaller overall grain size, as well as, a reduction in manufacturing cost due to relatively high cost of ruthenium. The increased coercivity also allows the use of a thinner Cr alloy underlayer, which also results in smaller overall grain size. Another benefit lies in the fact that the pre-seed layer provides additional thermal conductivity, which could help prevent thermal erasures on a glass disk. In the preferred embodiment, an onset layer is used with an optimal concentration of Cr and an optimal thickness adapted to increase coercivity and improve the Signal-to-Noise Ratio (SNR).

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patentapplication, Ser. No. 09/500,710, filed on Feb. 9, 2000, and entitled“NON-METALLIC THIN FILM MAGNETIC RECORDING DISK WITH PRE-SEED LAYER”.

BACKGROUND OF THE INVENTION

[0002] The use of a RuAl seed layer, which is included in the preferredembodiment discussed below, is described in a commonly assigned,co-pending U.S. patent application with Ser. No. 09/295,267. The use ofan onset layer, which is included in the preferred embodiment discussedbelow, is described in a commonly assigned, co-pending U.S. patentapplication with Ser. No. 08/976,565 entitled “Thin Film Disk with OnsetLayer.” U.S.P.T.O application Ser. No. 09/020,151, entitled “THIN FILMMAGNETIC DISK HAVING REACTIVE ELEMENT DOPED REFRACTORY METAL SEED LAYER”is mentioned below.

[0003] 1. Field of the Invention

[0004] This invention relates generally to the field of thin filmmaterials used in magnetic disks for data storage devices such as diskdrives. More particularly the invention relates to the use of animproved onset layer in between an underlayer and magnetic layer on athin film disk.

[0005] 2. Background of the Invention

[0006] The magnetic recording disk in a conventional drive assemblytypically consists of a substrate, an underlayer consisting of a thinfilm of chromium (Cr) or a Cr alloy, a cobalt-based magnetic alloydeposited on the underlayer, and a protective overcoat deposited on themagnetic layer. A variety of disk substrates such as NiP-coated AlMg,glass, glass ceramic, glassy carbon etc., are used. Disks that arecommonly available in the market are made with an AlMg substrate onwhich a layer of amorphous NiP is electrolessly deposited. While acoating on the substrate is important because such a coating givesuniform magnetic read-back signals during the course of a diskrevolution, the process of electroless deposition of NiP on an AlMgsubstrate has several disadvantages, one of them being the fact thatelectroless deposition is a wet process. The wet nature of the processnecessitates that it be performed quite separately from the sputteringprocess by which the remainder of the layers in a magnetic recordingdisk is deposited. A NiP layer has other disadvantages too. Forinstance, with a NiP layer, it is difficult to achieve the smoothnessand uniformity in the NiP surface of the magnetic recording disk, whichis a prerequisite for the high densities required in current diskdrives. Yet another problem associated with the NiP surface iscorrosion. The NiP surface also tends to limit the processingtemperatures because of its tendency to become magnetic if heated beyonda certain point.

[0007] Further, in cases where a non-metallic substrate such as glass ischosen, the conventional NiP coating is not preferable for use on glassas pre-seed layer for many reasons including those noted above. In suchcases, the non-metallic substrate disks typically have a so called “seedlayer” sputter deposited onto the substrate between the substrate andthe Cr-alloy underlayer. The selection of the seed layer allows theperformance of non-metallic substrates to exceed the magnetic recordingcharacteristics of NiP/AlMg disks because the seed layer of the magneticdisk drive influences nucleation and growth of the underlayer which inturn affects the recording characteristics of the magnetic layer.Several materials have been proposed in published papers for seed layerssuch as: Al, Cr, CrNi, Ti, Ni₃P, MgO, Ta, C, W, Zr, AlN and NiAl onglass and non-metallic substrates. (See for example, “Seed Layer induced(002) crystallographic texture in NiAl underlayers,” Lee, et al., J.AppI. Phys. 79(8), 15 April 1996, p.4902ff). In a single magnetic layerdisk, Laughlin, et al., have described use of a NiAl seed layer followedby a 2.5 nm thick Cr underlayer and a CoCrPt magnetic layer. The NiAlseed layer with the Cr underlayer was said to induce the [1{overscore(01)}0] texture in the magnetic layer. (“The Control andCharacterization of the Crystallographic Texture of Longitudinal ThinFilm Recording Media,” IEEE Trans. Magnetic. 32(5) September 1996,3632). In one of the related applications noted above, the use of RuAlfor a seed layer is disclosed.

[0008] A Cr underlayer is mainly used to influence such microstructuralparameters as the preferred orientation (PO) and grain size of thecobalt-based magnetic alloy forming the onset layer. When the Crunderlayer is deposited at elevated temperature on a NiP-coated AlMgsubstrate a [100] PO is usually formed. A PO of the underlayer promotesthe epitaxial growth of [1{overscore (12)}0] PO of the hcp cobalt (Co)alloy forming the onset layer, thereby improving the in-plane magneticperformance of the disk for longitudinal recording. The [1{overscore(12)}0] PO refers to a film of hexagonal structure whose (1{overscore(12)}0) planes are predominantly parallel to the surface of the film.Since nucleation and growth of Cr or Cr alloy underlayers on glass andmost non-metallic substrates differ significantly from those onNiP-coated AlMg substrates, different materials and layer structures areused on glass substrate disks to achieve optimum results.

[0009] The use of an onset layer has already been described in U.S. Pat.No. 5,736,262 in which a wide range of CoCr compositions were claimedincluding both ferromagnetic and non-magnetic alloys. The patentmentions that a non-magnetic onset layer is preferred because such anon-magnetic onset layer insures that the onset layer does not in anyway contribute towards the magnetic properties of the disk. U.S. Pat.No. 5,922,442 specifies an onset layer with a defined saturationmagnetization. However, the patent is silent about the desired Crconcentration of the onset layer. Since a magnetic layer of CoPtCrBalloy is difficult to grow epitaxially on a Cr alloy underlayer, thereexists a need for an onset layer, which is conducive for the growth ofmagnetic layers.

[0010] The design of magnetic disks has advanced rapidly in recent yearsand, even 1 dB improvement in the Signal-to-Noise Ratio (SNR) is nowconsidered quite significant. Recording density of magnetic disks ashigh as 30 to 40 gigabits per square inch has been achieved in theindustry. However, this density has only been achieved in the laboratoryand the density found in state of the art commercially available diskdrives is far below this value. The density of a disk is also dependenton the thermal stability of the recorded information on the disk becausea commercially viable disk drive must be capable of maintaining thestored information for periods of time measured in years.

SUMMARY OF INVENTION

[0011] The thin film disk of the invention includes a thin film pre-seedlayer of amorphous or nanocrystalline structure. The pre-seed layer,which may be chromium-tantalum CrTa or aluminum-titanium (AlTi) oraluminum tantalum (AlTa) is deposited prior to a first crystallinelayer. Although the pre-seed layer may be amorphous or nanocrystalline,for brevity it will be referred to herein as amorphous which is intendedto encompass a nanocrystalline structure. In the preferred embodiment ofthe present invention, a pre-seed layer is sputtered onto a non-metallicsubstrate such as glass, followed by a ruthenium-aluminum (RuAl) seedlayer with B2 structure. The use of the pre-seed layer improves grainsize and its distribution, in-plane crystallographic orientation,coercivity (Hc) and SNR. In a preferred embodiment of the presentinvention, the pre-seed layer is followed by a RuAl seed layer, a Cralloy underlayer, an onset layer and a magnetic layer. The amorphouspre-seed layer also allows use of a thinner RuAl seed layer whichresults in smaller overall grain size, as well as, a reduction inmanufacturing cost due to relatively high cost of ruthenium. Theincreased coercivity also allows the use of a thinner Cr alloyunderlayer, which also results in smaller overall grain size. Anotherbenefit lies in the fact that the pre-seed layer provides additionalthermal conductivity, which could help prevent thermal erasures on aglass disk. In accordance with a preferred embodiment of the presentinvention, an onset layer is used with an optimal concentration of Crand an optimal thickness adapted to increase coercivity and improve theSignal-to-Noise Ratio (SNR).

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a top view of a disk drive illustrating the structuralcomponents of a disk drive with a rotary actuator as used in the presentinvention.

[0013]FIG. 2 is a diagram illustrating the layer structure of a thinfilm magnetic disk according to a preferred embodiment of the presentinvention.

[0014]FIG. 3 is a graph illustrating the x-ray diffraction data forvarious samples of a magnetic disk drive showing the structuralvariations of CrTa with changes in composition.

[0015]FIG. 4 is a graphical illustration showing x-ray diffraction datafor samples of thin film magnetic disks showing the structuralvariations of materials with changes in thickness of a CrTa₅₀ pre-seedlayer according to the invention.

[0016]FIG. 5 is a graphical illustration showing x-ray diffraction datafor samples of thin film magnetic disks showing the structuralvariations of the materials with changes in thickness of a AlTi₅₀ thinfilm layer according to a preferred embodiment of the present invention.

[0017]FIG. 6 is a graphical illustration showing remanent coercivityagainst onset layer thickness to demonstrate an optimal thickness forthe onset layer.

[0018]FIG. 7 is a graphical illustration showing remanent magnetizationtimes thickness against onset layer thickness to demonstrate an optimalthickness for the onset layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019]FIG. 1 is a top view illustrating a disk drive with a rotaryactuator in which a thin film disk according to a preferred embodimentof the present invention may be used. The disk drive system includes oneor more magnetic recording disks 111 mounted on a spindle 112, which isrotatable by an in-hub electrical motor (not shown). An actuatorassembly 115 supports a slider 120, which contains one or moreread/write heads. The actuator assembly 115 is composed of a pluralityof actuators and sliders arranged in a vertical stack with the actuatorssupporting the sliders being in contact with the surfaces of the diskswhen the disks are not rotating or being unloaded to avoid contact. Avoice coil motor (VCM) 116 moves the actuator assembly 115 relative tothe disks by causing the assembly to pivot around a shaft 117. Theread/write heads are typically contained in air bearing sliders adaptedfor flying above the surface of the disks when rotating at a sufficientspeed. During the operation of the disk drive, if the sliders fly abovethe disks the VCM moves the sliders in an arcuate path across the disksso as to allow the heads to be positioned to read and write magneticinformation from the circular tracks which are formed in the data area114. The data area is coated with thin films as described below.Electrical signals to and from the heads and the VCM are carried by aflex cable 118 to drive electronics 119. When the disk drive is notoperating and during such periods of time as when the rotation of thedisks is either starting or stopping, the sliders may either be removedfrom the disks using load/unload ramps (not shown) or parked in physicalcontact with the surface of the disks in a landing zone or contactstart/stop (CSS) area 113 which is not used for data storage even thoughthe magnetic coating extends over this area. If the sliders are unloadedfrom the disks during non-operation, there is no need to have a CSS areaand more of the disk becomes available for data storage. Although thedisk drive has been described with air bearing sliders the disk of thepresent invention may easily be used in other storage devices havingnear contact, or contact recording sliders.

[0020]FIG. 2 is a diagram illustrating the layer structure of a thinfilm magnetic disk according to a preferred embodiment of the presentinvention. The thin film layers are deposited onto at least one andpreferably both planar surfaces of the magnetic disk to form the datarecording area. The substrate 10 may be made of glass or any othersuitable material.

[0021] A CrTa or AlTi or AlTa pre-seed layer 12 is first deposited ontothe substrate. The pre-seed layer is deposited by conventional DCmagnetron sputtering. The relative composition of Cr versus Ta isselected to produce a film with an amorphous or nanocrystallinestructure. The use of a pre-seed layer improves the media coercivity fora film structure with very thin RuAl seed layer and ultra-thin Cr alloyunderlayer.

[0022] The RuAl seed layer 14 is next deposited directly onto thepre-seed layer. The seed layer could also be a “double layer” with alayer of RuAl followed by a layer of NiAl, for example. This doublelayer configuration could result in cost savings by reducing the amountof Ru required. Ru is an expensive element, so a reduction in therequired quantity of Ru will reduce the costs. In the double layerstructure the RuAl seed layer establishes the grain size and orientationand the subsequently deposited NiAl follows the established patterns. Anunderlayer 16 is next deposited onto the seed layer and is comprised ofa non-ferromagnetic material such as a chromium alloy e.g. CrV or CrTi.The underlayer is followed by a Co-alloy onset layer 18 and a CoPtCrBmagnetic layer 20. The use of an onset layer 18 is described in acommonly assigned, co-pending U.S. patent application with Ser. No.08/976,565. Typically, the onset layer material is selected in part forits lattice match with the underlayer. Lattice parameters, which areintermediate between that of the underlayer 16 and the magnetic layer 20may strengthen the epitaxy in the desired orientation. In accordancewith a preferred embodiment of the present invention, the onset layer 18is of hexagonal close packed (hcp) structured material, which isferromagnetic. Materials, which are suitable to form the onset layer,include cobalt-chromium alloys with Cr composition around 31 at. %. Inaccordance with a preferred embodiment of the present invention, theonset layer is between 0.5 to 2.5 nm thick.

[0023] A preferred embodiment of the present invention has a magneticlayer 20 deposited on the onset layer 18. The magnetic layer is an alloyof cobalt, which typically contains platinum and chromium and maycontain additional elements such as tantalum or boron, e.g. CoPtCrTa orCoPtCrB. A typical magnetic layer might comprise 12 to 20 at. %platinum, 16 to 20 at. % chromium and 6 to 10 at. % boron with cobaltforming the remainder of the magnetic layer. The magnetic layer can bein the thickness range of 5-30 nm thick with 10-20 nm being thepreferred thickness range. The use, composition and thickness of anovercoat 22 on top of the magnetic layer are not critical in practicingthe invention, but a typical thin film disk might use an overcoat lessthan 15 nm thick.

[0024] While the compositions listed above have been given withoutregard to contamination percentages, it is known to those skilled in theart that some contamination is normally, if not always, present in thinfilms. Sputtering targets are typically specified as 99.9% or greaterpurity, but the resulting films may have much lower purity due tocontamination in the sputtering chamber or other factors. For example,contamination by air in the chambers might result in measurable amountsof oxygen and/or hydrogen being incorporated into the film. It is alsoknown that some small amount of oxygen is normally found in Cr targetsand in the resulting Cr layer. It is also possible for small amounts ofthe working gas in the sputtering system, e.g. argon, to be incorporatedinto a sputtered film. Contamination levels were not specificallymeasured in the disk samples described and, therefore, were assumed tobe within normal ranges for sputtered thin film disks expected by thoseskilled in the art.

[0025] The thin film disk made according to the invention can be usedfor storing data in typical disk drives using either magnetoresistive orinductive heads and can be used in contact recording or with flyableheads. The read/write head is positioned over the rotating disk in astandard manner to either record or read data.

[0026] In general, the application of some type of seed layer on glassand other alternate substrates to control nucleation andcrystallographic orientation of Cr (or Cr alloys), and thereby themagnetic Co-alloy layer is well known. U.S. Pat. No. 5,789,056 disclosedthat the use of a very thin seed layer and underlayer on glass mediareduce the grain size of magnetic alloy substantially, thereby improvingSNR. By applying different compositions of seed layers,crystallographically textured (112) or (100) Cr layer can be deposited.By sputtering Co-alloys on these Cr underlayers, textures of either(1{overscore (01)}0) or (1{overscore (12)}0) in the magnetic layer canbe achieved. For high deposition rate sputtering, it has been found thatthe application of an amorphous TaN seed layer on glass induces a (100)orientation in the subsequently grown Cr underlayer, which promotes astrong (1{overscore (12)}0) orientation in the Co-alloy layer. (SeeU.S.P.T.O application Ser. No. 09/020,151, filed: Feb. 6, 1998, THINFILM MAGNETIC DISK HAVING REACTIVE ELEMENT DOPED REFRACTORY METAL SEEDLAYER). However, the formation of TaN layer requires a reactiveatmosphere in the sputtering chamber and, therefore, increases thedifficulty encountered during manufacturing. The (1{overscore (12)}0)texture of a Co-alloy layer can also be obtained by depositing arelatively thick RuAl seed layer, but the high cost of RuAl sputteringtargets is a major drawback for its use in large scale manufacturing.The use of a CrTa pre-seed layer 12 allows the use of a very thin RuAlseed layer (and thus reduces cost) and an ultra-thin Cr (or Cr-alloys)underlayer on a glass substrate, which in turn enables the subsequentgrowth of strong (1{overscore (12)}0) oriented Co-alloy onset layer withcontrolled smaller grain size.

[0027] In accordance with a preferred embodiment of the presentinvention, the pre-seed layer 12 is sputter deposited onto a glasssubstrate 10 followed by a thin RuAl seed layer 14, an ultra-thinCr-alloy underlayer 16, a Co-alloy onset layer 18 and a CoPtCrB magneticlayer 20. In order to enhance the lattice match between the RuAl seedlayer 14 and the Cr-alloy underlayer 16, CrTi or CrMo alloys arepreferred to form the underlayer. A CrMo underlayer is also advantageousbecause it helps render the SNR less sensitive to changes in theunderlayer thickness. Magnetic properties and SNR data for disks withand without a CrTa and AlTi pre-seed layer are listed in Table-1 for acomparison. TABLE 1 Hc SoNR Disk Structure (Oe) Mrt S* (dB) 1RuAl₅₀/CrTi₁₀/CoCr₃₇/CoPt₁₁Cr₂₀B₇ 3040 0.375 0.66 27.6 2CrTa₅₀/RuAl₅₀/CrTi₁₀/CoCr₃₇/ 3660 0.420 0.80 27.7 CoPt₁₁Cr₂₀B₇ 3NiAl₅₀/CrV₂₀/CoCr₃₇/CoPt₁₀Cr₂₀B₆ 3400 0.420 0.78 26.4 4AlTi₅₀/RuAl₅₀/CrTi₁₀/CoCr₃₇/ 3500 0.430 0.81 27.7 CoPt₁₁Cr₂₀B₇

[0028] The data in table 1 shows that although disks 1 and 2 have acomparable SoNR, disk 2 with a CrTa₅₀, i.e., 50 at. % Ta, pre-seed layerexhibits a substantially higher Hc and coercive squareness S*. At thesame Mrt (remanent magnetization times thickness) as disk 3, disk 2gives rise to a SoNR improvement of 1.3 dB as compared to the NaAl₅₀seed layer structure of disk 3. Disk 4, which was made with an AlTi₅₀pre-seed layer, also shows an improved performance over the prior artNiAl₅₀ (disk 3). The high coercivity and squareness achieved with CrTaand AlTi pre-seed layers is a result of creating the enhanced RuAl <100>and in-plane Co <1{overscore (12)}0> textures.

[0029]FIG. 3 is a graph illustrating the X-ray diffraction data forvarious samples of a magnetic disk drive showing the structuralvariations of CrTa with different changes in composition. The figureshows spectra for a set of film structures with CrTa₅₀, CrTa₂₀, CrTa₁₀,CrTa₂ and Ta only pre-seed layers. No significant peaks appear for theCrTa₅₀ film indicating that this composition results in a substantiallyamorphous or nanocrystalline film. Both the pure Ta and CrTa₂₀ filmsresult in significant diffraction peaks indicating crystallinestructure.

[0030]FIG. 4 is a graphical illustration showing X-ray diffraction datafor samples of thin film disks made with varying thickness of CrTa₅₀pre-seed layers ranging from 0 to 45 nm. The results are consistent upto a thickness of 60 nm. Throughout the X-ray spectra no crystallineCrTa diffraction peaks are observed, thus confirming the amorphousnature of the pre-seed layer. As is known to those of ordinary skill inthe art, the use of RuAl seed layer on glass substrate creates a Cr<200> texture which leads to a <1{overscore (12)}0> texture in theCo-alloy layer. For a structure without the CrTa pre-seed layer, all thediffraction peak intensities are very weak, indicating poor structuralintegrity due to the deposition of very thin RuAl seed layer and CrTiunderlayer directly on glass. By depositing a CrTa pre-seed layer,substantial enhancements of RuAl (100), (200), CrTi (200) and Co-alloy(1{overscore (12)}0) diffraction peaks are observed, indicating asignificant improvement of the C-axis in-plane orientation.

[0031]FIG. 5 shows X-ray diffraction plots for disks made with 0, 150,300 and 450 Angstroms of thicknesses of AlTi₅₀ pre-seed layers. Theresults are believed to be valid down to a thickness of 100 Angstroms(A), i.e. 10 nm. The disks had RuAl seed layers, CrTi underlayers andcobalt alloy magnetic layers. The graph shows that the preferredorientations of RuAl (100), RuAl (200), CrTi (200) and Co (1{overscore(12)}0) strengthen with increased thickness of the AlTi pre-seed layer.As shown in FIG. 3 the preferred composition for the CrTa pre-seed layeris CrTa₅₀. The behavior of AlTi is similar to CrTa in this respect, sothe preferred composition for an AlTi is also 50-50.

[0032] Table 2 summarizes the values of full width half-maximum (FWHM)derived from the RuAl (200) and Co (1{overscore (12)}0) peaks. It isclear that the much smaller FWHM values are measured for the filmstructure with either CrTa₅₀ or AlTi₅₀ pre-seed layers. The smaller FWHMvalues indicate a high degree of in-plane texture and less dispersion of[1{overscore (12)}0] preferred orientation of the hexagonal Costructure. TABLE 2 RuAl (200) Co(11{overscore (2)}0) Film structure(°FWHM) (°FWHM) RuAl₅₀/CrTi₁₀/CoCr/CoPtCrB 17.6 12.8CrTa₅₀/RuAl₅₀/CrTi₁₀/CoCr/CoPtCrB 5.8 5.2AlTi₅₀/RuAl₅₀/CrTi₁₀/CoCr/CoPtCrB 8.7 6.8AlTa₂₀/RuAl₅₀/CrTi₁₀/CoCr/CoPtCrB 11.6 8.8

[0033] It is also known that the poor thermal conductivity of glasssubstrates can cause the recorded data bits to be thermally erased. Theuse of a relatively thick CrTa pre-seed layer could potentially havesome advantage in addressing the thermal erasure issue related to aglass disk medium.

[0034] Use of sputtered NiP pre-seed layer together with a Cr sub-seedlayer and a NiAl seed layer was published by Chen, Yen, Ristau, Ranjan.(“Effect of Cr sub-seed layer thickness on the crystallographicorientation of Co-alloy recording media on glass,” IEEE Trans. Magn. 35,pp. 2637-2639 (1999). Their results showed that (1{overscore (12)}0)Co-alloy texture can be generated for thicker (>45 Å) Cr sub-seedlayers, but the SNR was poor due to larger grain size. For thin Crsub-seed layers, the use of the NiAl seed layer induces (1{overscore(01)}0) Co-alloy texture.

[0035] RuAl tends to form the B2 (cesium chloride) structure in asputtered thin film. Small amounts of other materials could conceivablybe added to RuAl without disrupting the critical B2 structure. The B2structure is an ordered cubic structure that can be described as twointerpenetrating simple cubic lattices where, for RuAl, Al atoms occupyone lattice and Ru atoms the other. RuAl has a lattice constant, whichis close to that of Cr even though Cr has a bcc structure. RuAl tends toform smaller grain size than Cr due to the strong bonding between the Ruand Al atoms, which reduces atomic mobility during deposition.

[0036] The role of the RuAl seed layer of the preferred embodiment ofinvention is to ultimately control the orientation, grain size and grainsize distribution of magnetic grains. The grain size and orientationachieved in a RuAl seed layer is propagated into the magnetic layerthrough epitaxial growth of properly selected subsequent layersincluding the magnetic layer. Whereas the traditional thin film magneticdisk has only three layers e.g., underlayer, magnetic layer andovercoat, the trend in the industry is towards using additional layers.The terminology for these additional layers has not become standardized,but in a descriptive sense, there may be pre-seed layers 12, seed layers14, one or more underlayers 16, non-magnetic or magnetic onset layers18, a plurality of magnetic layers 20, which may or may not have spacerslayers separating them. In addition, what is called the “substrate” 10may in fact be multilayered material. In this context of proliferatinglayers, the RuAl seed layer can be effective in achieving the beneficialresults described herein so long as it is deposited in the B2 structureand ahead of the magnetic layer. Thus, the RuAl seed layer in thepreferred embodiment is intended to be the first non-amorphous layer toinfluence crystallographic orientation and grain size of subsequentlydeposited magnetic material.

[0037] In accordance with a preferred embodiment of the presentinvention, the CrTa or AlTi pre-seed layer 12 is sputter deposited ontothe substrate (which may already have thin films on it) from targetscomposed substantially of (a) CrTa and preferably CrTa₅₀, or (b) AlTiand preferably AlTi₅₀. The RuAl seed layer 14 is deposited onto thepre-seed layer by standard sputtering techniques.

[0038] In accordance with a preferred embodiment of the presentinvention, a ferromagnetic CoCr alloy is used as an onset layer 18between the underlayer 16 and the magnetic layer 20 because CoPtCrBmagnetic alloys are difficult to grow epitaxially on Cr alloyunderlayers especially with higher boron concentrations. The onset layer18 has a Cr concentration in the range of 28-33 at. %.

[0039] Since the Cr concentration in the CoCr onset layer 18significantly affects the magnetic properties of the disk, the Crconcentration in the CoCr onset layer is optimized in accordance with apreferred embodiment of the present invention. An addition of Cr inexcess of the optimal concentration results in the impairment of SNR. Onthe other hand, if Cr concentration is very low, the onset layer 18becomes magnetic in character and lowers the magnetocrystallineanisotropy (Ku) of the onset layer. Therefore, the Cr concentration ofthe onset layer 18 must be optimized for use with CoPtCrB magneticlayers at a concentration range of 28-33 at. % in order to obtain thebest SNR performance. Since the CoCr onset layer is ferromagnetic inthis concentration range, the thickness of the onset layer 18 needs tobe maintained at an optimum range of 0.5 to 2.5 nm so as to achievemaximum coercivity and an improved SNR. TABLE 3 Remanent Coercivity MrTSNR at 280 kbpi Onset Layer (Oe) (memu/cm2) (dB) CoCr₃₇ 3660 0.46 29.7CoCr₃₁ 3490 0.45 30.4

[0040] A comparison of changes in magnetic properties observed indifferent onset layers with varying concentrations of Cr is presented intable 3. Both of the onset layers have a thickness of 4 nm. Table 3illustrates coercivity and SNR results for a two-layered CoPtCrB alloy(12 nm CoPtl₄Cr₂₀B₈ deposited on 12 nm COPtgCr₂₀B₈) for two differentonset layers CoCr₃₇ and CoCr₃₁.

[0041] As is seen in table 3, the CoCr₃₁ onset layer displays decreasedcoercivity and decreased MrT (Remanent magnetization times thickness) ascompared to the CoCr₃₇ onset layer. However, an improvement in SNR isobserved for the CoCr₃₁ onset layer as compared to the CoCr₃₇ onsetlayer. TABLE 4 Onset Layer Remanent Coercivity MrT SNR at 400 kbpiThickness (nm) (Oe) (memu/cm2) (dB) 4 3873 0.32 23.2 2 4161 0.31 24.2

[0042] A comparison of changes in magnetic properties observed indifferent onset layers 18 with varying thicknesses of 2 and 4 nm ispresented in table 4, which contains data for a CoPt₈Cr₂₀B₈ alloy usinga CoCr₃₁ onset layer. As shown in table 4, an increase in coercivity andimproved SNR is observed for the thinner onset layer. The thicker onsetlayer causes a decrease in magnetocrystalline anisotropy of the magneticlayer.

[0043]FIG. 6 is a graphical illustration showing the changes in remanentcoercivity (Hc) with varying onset layer thicknesses when thecomposition of the onset layer is the same. The thickness of themagnetic layer is maintained at 13 nm. As shown in FIG. 6, two onsetlayers with the same thickness but differing concentrations of Cr in thelayer display differences in properties such as coercivity (Hc). TheCoCr₃₃ onset layer displays a greater increase in remanent coercivity ascompared to the CoCr₂₈ onset layer, thickness of the two layers beingthe same.

[0044] Additionally, remanent coercivity of a magnetic disk increasesfurther with a reduction of thickness until a maximum coercivity (Hc) isachieved for a thinner onset layer at a thickness of approximately 1.5nm. In accordance with a preferred embodiment of the present invention,the Cr concentration of the onset layer is maintained above 28 at. % toallow the onset layer to be thick enough to initiate growth of theCo-alloy magnetic layer without significantly degrading the Hc of theonset layer.

[0045]FIG. 7 is a graphical illustration showing the changes in remanentmagnetization times thickness MrT for a particular composition of theonset layer 18. The thickness of the magnetic alloy is maintained at 13nm. As shown in FIG. 7, two onset layers of the same thickness butdifferent Cr concentrations display differences in MrT. The CoCr₂₈ onsetlayer displays a higher increase in MrT as compared to the CoCr₃₃ onsetlayer for the same thickness.

[0046] In accordance with a preferred embodiment of the presentinvention, high Cr concentrations in the CoCr onset layer do not inducethe best SNR performance. Likewise, a thick onset layer induces animpaired SNR performance because a thick onset layer contributes towardsthe magnetization which in turn results in an impaired SNR. Therefore,in accordance with a preferred embodiment of the invention, thinneronset layers 18 with thickness in the range of 0.5 to 2.5 nm and lowerCr concentration are used in the hard disk of the present invention.

[0047] While the preferred embodiments of the present invention havebeen illustrated in detail, it will be apparent to the one skilled inthe art that alternative embodiments of the invention are realizablewithout deviating from the scope and spirit of the invention.

What is claimed is:
 1. A thin film magnetic disk comprising: asubstrate; a pre-seed layer with an amorphous or nanocrystallinestructure; a non-magnetic ruthenium-aluminum (RuAl) seed layer depositedover the pre-seed layer; at least one non-magnetic underlayer depositedover the RuAl seed layer; at least one onset layer deposited over theunderlayer, wherein said onset layer is comprised of CoCr and whereinthe concentration of Cr is in the range of 28-33 at. %; and at least onemagnetic layer deposited over the onset layer.
 2. The disk of claim 1 ,wherein the onset layer is ferromagnetic in nature.
 3. The disk of claim1 , wherein the thickness of the onset layer is in the range of 0.5 to2.5 nm.
 4. The disk of claim 1 , wherein the pre-seed layer is CrTa andcontains approximately 50 at. % Ta
 5. The disk of claim 1 , wherein thepre-seed layer is AlTi and contains approximately 50 at. % Ti
 6. Thedisk of claim 1 , wherein the pre-seed layer is AlTa and containsapproximately 20 at. % Ta.
 7. The disk of claim 1 , wherein thethickness of the pre-seed layer is greater than or equal to 10 nm. 8.The disk of claim 1 , wherein the thickness of the pre-seed layer isless than or equal to 60 nm.
 9. The disk of claim 1 , wherein the RuAlseed layer is between 3 and 20 nm in thickness.
 10. The disk of claim 1, wherein the RuAl seed layer has a B2 structure.
 11. The disk of claim1 , wherein the RuAl seed layer has a <200> preferred orientation. 12.The disk of claim 1 , wherein the underlayer is a chromium alloycontaining approximately 10 at. % titanium.
 13. The disk of claim 1 ,wherein the underlayer comprises CrTi with a <200> preferredorientation.
 14. The disk of claim 1 , wherein the underlayer comprisesCrTi and is between 3 and 15 nm in thickness.
 15. The disk of claim 1 ,wherein the magnetic layer is comprised of a CoPtCrB alloy.
 16. A diskdrive comprising: a motor for rotating a spindle; a thin film magneticdisk mounted on the spindle; and an actuator assembly including a headfor writing magnetic information on the disk as it rotates, wherein saidthin film disk includes: a substrate; a pre-seed layer with an amorphousor nanocrystalline structure; a non-magnetic ruthenium-aluminum (RuAl)seed layer deposited over the pre-seed layer; at least one non-magneticunderlayer deposited over the RuAl layer; at least one onset layerdeposited over the underlayer, wherein said onset layer is comprised ofCoCr and wherein the concentration of Cr is in the range of 28-33 at. %;and at least one magnetic layer deposited over the onset layer.
 17. Thedisk drive of claim 16 , wherein the onset layer is ferromagnetic innature.
 18. The disk drive of claim 16 , wherein thickness of the onsetlayer is in the range of 0.5 to 2.5 nm.
 19. The disk drive of claim 16 ,wherein the pre-seed layer is CrTa and contains approximately 50 at. %Ta.
 20. The disk drive of claim 16 , wherein the pre-seed layer is AlTiand contains approximately 50 at. % Ti.
 21. The disk drive of claim 16 ,wherein the pre-seed layer is AlTa and contains approximately 20 at. %Ta.
 22. The disk drive of claim 16 , wherein the thickness of thepre-seed layer is greater than or equal to 10 nm.
 23. The disk drive ofclaim 16 , wherein the thickness of the pre-seed layer is less than orequal to 60 nm.
 24. The disk drive of claim 16 , wherein the RuAl seedlayer is between 3 and 20 nm in thickness.
 25. The disk drive of claim16 , wherein the RuAl seed layer has a B2 structure.
 26. The disk driveof claim 16 , wherein the RuAl seed layer has a <200> preferredorientation.
 27. The disk drive of claim 16 , wherein the underlayer isa chromium alloy containing approximately 10 at. % titanium.
 28. Thedisk drive of claim 16 , wherein the underlayer comprises CrTi with a<200> preferred orientation.
 29. The disk drive of claim 16 , whereinthe underlayer comprises CrTi and is between 3 and 15 nm in thickness.30. The disk drive of claim 16 , wherein the magnetic layer is comprisedof a CoPtCrB alloy.
 31. A method of manufacturing a thin film magneticdisk comprising the steps of: depositing a thin film pre-seed layer withan amorphous or nanocrystalline structure onto a substrate; depositing acrystalline ruthenium-aluminum (RuAl) seed layer over the pre-seedlayer; depositing at least one non-magnetic underlayer over the RuAlseed layer; depositing at least one onset layer over the underlayer,wherein said onset layer is comprised of CoCr and wherein theconcentration of Cr is in the range of 28-33 at. %; and depositing atleast one magnetic layer over the onset layer.
 32. The disk drive ofclaim 31 , wherein the onset layer is ferromagnetic in nature.
 33. Thedisk drive of claim 31 , wherein thickness of the onset layer is in therange of 0.5 to 2.5 nm.
 34. The disk drive of claim 31 , wherein thepre-seed layer is CrTa and contains approximately 50 at. % Ta.
 35. Themethod of claim 31 , wherein the pre-seed layer is CrTa and containsapproximately 50 at. % Ta.
 36. The method of claim 31 , wherein thepre-seed layer is AlTi and contains approximately 50 at. % Ti.
 37. Themethod of claim 31 , wherein the pre-seed layer is AlTa and containsapproximately 20 at. % Ta.
 38. The method of claim 31 , wherein thethickness of the pre-seed layer is greater than or equal to 10 nm. 39.The method of claim 31 , wherein the thickness of the pre-seed layer isless than or equal to 60 nm.
 40. The method of claim 31 , wherein theRuAl seed layer is between 3 and 20 nm in thickness.
 41. The method ofclaim 31 , wherein the RuAl seed layer has a B2 structure.
 42. Themethod of claim 31 , wherein the RuAl seed layer has a <200> preferredorientation.
 43. The method of claim 31 , wherein the underlayercomprises CrTi and is between 3 and 15 nm in thickness.
 44. The methodof claim 31 , wherein the underlayer is a chromium alloy containingapproximately 10 at. % titanium.
 45. The method of claim 31 , whereinthe underlayer comprises CrTi with a <200> preferred orientation. 46.The method of claim 31 , wherein the magnetic layer is comprised of aCoPtCrB alloy.